1) Field of the Invention
The present invention relates to a signal processing circuit for carrying out signal processing on an I (In-Phase) signal and a Q (Quadrature) signal having undergone quadrature detection and a quadrature demodulation apparatus and a method of estimating the error thereof which are suitable for use in a receiver of a communication system, or more preferably in a receiver of a radio wave communication system.
2) Description of the Related Art
FIG. 8 is a block diagram showing a general arrangement of a receiver 100. As shown in FIG. 8, the receiver 100 has an antenna 101 and a radio wave signal sent from a transmitter not shown can be received by the antenna 101. The signal received by the antenna 101 is placed under control of an AGC (Automatic Gain Control) amplifier 102 so that the signal has a constant gain. This signal is supplied to a band-pass filter 103 in which unnecessary wave components are removed and then subjected to a quadrature demodulation in a quadrature demodulation unit 104. In this way the signal is converted into a baseband signal.
The quadrature demodulation unit 104 is arranged to include a zero-degree hybrid (0 deg. HYB) 104a for dividing a radio wave frequency signal fRF passing through the band-pass filter 103 into a couple of signal flows, a local oscillator 104b for generating a local signal having a frequency equal to that of the received demodulated signal, a ninety-degree hybrid (90 deg. HYB) 104c for producing from the aforesaid local signal from the local oscillator 104b a couple of local signals having a phase shifted by 90 deg. with respect to each other, and outputting them to two mixers 104i and 104q. 
That is, the two mixers 104i and 104q multiply the couple of radio wave frequency signal deriving from the branching by the aforesaid zero-degree hybrid 104a with a couple of local signals generated from the ninety-degree hybrid 104c, respectively. Thus, an I-signal and a Q-signal, i.e., a pair of baseband signals having a quadrature property relative to each other, can be generated from the mixers 104i and 104q, respectively.
Subsequently, in the receiver 100 shown in FIG. 8, the I-signal and the Q-signal (hereinafter sometimes referred to as I/Q signals) generated from the quadrature demodulation unit 104 are supplied to low-pass filters 105i and 105q in which high frequency components are removed therefrom, respectively. Then, the signals are supplied to A/D (analog-to-digital) converters 106i and 106q in which the signals are converted into digital signals, respectively. Thereafter, the digital signals supplied from the A/D converters 106i and 106q are supplied to a digital demodulation processing unit 107 in which data demodulation is carried out.
Meanwhile, it is known that the digital signals supplied to the digital demodulation processing unit 107 suffer from amplitude error or phase error caused by inherent characteristics of devices constituting the quadrature demodulation unit 104 or characteristics of the low-pass filters 105i and 105q and the A/D converters 106i and 106q. 
For example, one of device components arranged as the zero-degree hybrid 104a cannot always have a property of uniform signal component distribution to the baseband of the I-signal side and the Q-signal side, due to the inherent device characteristic. For this reason, amplitude error tends to occur between the I-signal and the Q-signal.
In more concretely, as shown in FIG. 11b, the Q-signal distributed by the zero-degree hybrid 104a can have a uniform gain in a signal band BW. However, as for the I-signal distributed in the similar manner as shown in FIG. 11a, the I-signal tends to have a gain lowered in a relatively high frequency band zone as compared with a relatively low frequency band zone. As a consequence, the amplitude error (i.e., the gain difference) of the I/Q signals will be placed under influence of the amplitude error of the I-side signal. Thus, as shown in FIG. 11c, the high frequency band zone has an amplitude value lowered as compared with the low frequency band zone.
Furthermore, one of device components arranged as the zero-degree hybrid 104a and the ninety-degree hybrid 104c cannot always have a property of uniform distribution of a signal having an ideal angle relative to the input, due to the inherent device characteristic. For this reason, phase error tends to occur between the I-signal and the Q-signal.
In more concretely, as shown in FIG. 12a, the zero-degree hybrid 104a tends to distribute the radio wave frequency signal which is shifted relative to the ideal zero-degree. Therefore, for example, the high frequency component tends to have error proceeding relative to the zero-phase in the radio frequency signal band, and the low frequency component tends to have error delaying relative to the zero-phase. Furthermore, as shown in FIG. 12b, also the ninety-degree hybrid 104c tends to generate a couple of local signals shifted from the ideal 90 degrees. For this reason, as shown in FIG. 12c, the phase error between the I-signal and the Q-signal which serve as outputs of the quadrature demodulation unit 104 will become one deriving from summation of phase errors of the zero-degree hybrid 104a and the ninety-degree hybrid 104c at the mixers 104i and 104q. 
The above-described quadrature demodulation unit 104 will generate the I-signal and the Q-signal which are left having the summation of the aforesaid amplitude errors and the phase errors.
Furthermore, the quadrature demodulation unit 104 has separated channels for the I/Q signals in the later stage thereof and each of the channels has a series of components (see reference numerals 105i, 106i, 105q, 106q). These components have variation in gain and this variation causes an electric power value difference in the I/Q signals and this electric power value difference will also cause amplitude error.
If the received signal component containing the above-described received amplitude error and the phase error (such kind of errors will sometimes take amplitude-phase error as a generic name) is subjected to data demodulation processing in the digital demodulation processing unit 107, then difficulty will be brought about in improvement of the BER (bit error rate) characteristic.
If the receiver 100 is requested to handle a signal having a relatively low S/N ratio (signal-to-noise ratio), only small influence will be caused from the amplitude error and the phase error. However, if the receiver 100 is requested to handle a signal in a manner of spread spectrum coding or to transmit a signal having a format necessitating a high S/N ratio such as of multilevel coding signal, the amplitude-phase error components will look like a noise, with the result that relatively large influence will be caused on the BER ratio.
Accordingly, in order to achieve improvement in the receiver at the characteristic of the data demodulation processing in the digital demodulation processing unit 107, it is requested to eliminate the above-described amplitude-phase error component. As a technology for solving a problem of the above-described amplitude error or the phase error, there can be introduced known technology disclosed in the following Patent document 1 and Patent document 2.
According to the technology disclosed in Patent document 1, as for a signal having undergone digital demodulation processing in a demodulation circuit, a received signal quality is detected from a signal before error correction processing and a signal having undergone error correction processing for internal code and external code. Thereafter, in order for correcting the amplitude error and the phase error of the I/Q signals, feedback control is effected on the amount of shift of a ninety-degree shifter and the characteristic of a low-pass filter.
Further, Patent document 2 discloses in FIG. 1 thereof an arrangement of a receiving apparatus that includes detectors (see reference numerals 102, 103) for detecting received signals deriving from branching-into-two-signal process based on reference signals having a phase difference of 90 deg., a level detector (see reference numeral 112) for detecting an output level of the detectors, a level control signal generator (see reference numeral 114) for generating a level control signal in accordance with the output of the level detector, and a level controllers (see reference numerals 110 and 111) for varying the reference signal level based on the output of the level control signal generator and controlling the output of the detector.
Meanwhile, one of recent technologies known as OFDM (Orthogonal Frequency Division Multiplexing) is drawing attention as a technology that can realize a wide band transmission effectively using a small frequency range and improve the efficiency of frequency band utilization. The OFDM uses frequency orthogonality thereby allows signals to be overlapped on a frequency axis.
In the OFDM, a carrier is divided into a plurality of subcarriers and data is transmitted as parallel data, whereby the symbol rate thereof can be suppressed, transmission rate can be increased and the symbol length can be kept long. Then, the orthogonal m (#1 to #m) subcarriers are arrayed alternately as shown in FIG. 9 for utilization. Thus, interference among the subcarriers can be eliminated, and the subcarriers can be arrayed in a relatively narrow band width BW at a high density.
In more concretely, in a case of transmitter employing the OFDM technology, an IFFT (Inverse Fast Fourier Transform) is effected on the parallel data to transform data in the subcarrier frequency domain into data in the time domain, and thereafter the transformed data is subjected to a quadrature demodulation with a radio frequency signal fRF, for example, to carry out radio transmission.
When a receiver receives the signal transmitted by way of radio transmission, the receiver effects quadrature demodulation on the received signal, and thereafter carries out a processing of Fast Fourier Transform to obtain I/Q data pieces of respective subcarriers as the parallel data of original transmission.
FIG. 10 is a block diagram showing a receiver 110 employing the aforesaid OFDM technology. When a received signal is introduced into the quadrature demodulation unit 104 through the antenna 101, the AGC amplifier 102 and the band-pass filter 103, the quadrature demodulation unit 104 carries out quadrature demodulation processing. Then, the low-pass filters 105i and 105q remove high frequency components from the received signals. The A/D converters 106i and 106q convert the received signals into digital signals. This set of processing is carried out in the same manner as that of FIG. 8. However, the arrangement of FIG. 10 is particularly equipped with an FFT (Fast Fourier Transform) unit 108 between the A/D converters 106i and 106q and the digital demodulation processing 107.
That is, the FFT unit 108 is arranged to convert the I/Q signals supplied from the A/D converters 106i and 106q into I/Q signals (I1, Q1, . . . ,Im, Qm) for each of the plural number (m) of subcarriers and then output the converted signals to the digital demodulation processing unit 107.
Patent Document 1: Japanese Patent Laid-Open No. 2003-8674
Patent Document 2: Japanese Patent Laid-Open No. HEI 11-252188
However, according to the technology disclosed in the above-introduced Patent Document 1, the signal processing thereof will suffer from influence caused by phase error until the error correction processing comes to have stable accuracy and the feedback control goes into a stable operation mode. For this reason, it is difficult for the received signal to be subjected to phase error correction and amplitude error correction for a certain period of time in which the feedback control goes into a stable operation mode.
Further, according to the technology disclosed in the above-introduced Patent Document 2, there is no discloser contained about a technology which can handle the phase error correction. Therefore, when more improvement is tried in bit error rate, the technology will encounter certain difficulties. Furthermore, since the technology includes a feedback control, it is difficult for the received signal to be subjected to amplitude error correction for a certain period of time in which the feedback control goes into the stable operation mode.
In the receiver 110 employing the OFDM shown in FIG. 10, the quadrature demodulation unit 104 carries out quadrature demodulation processing by using an fRF signal having a single frequency on a signal which is transmitted from a transmitter employing the OFDM technology in a manner described above. That is, owing to the quadrature demodulation processing, it becomes possible to obtain an I-signal and a Q-signal constituting signals which derive from conversion from the frequency domain data into a time domain data by the IFFT processing.
That is, in the above arrangement, the quadrature demodulation is effected over the all frequency domain having the plurality of subcarrier frequency band superposed on one another by using the signal fRF having a common frequency. Therefore, even if the amplitude error correction and the phase error correction are effected on the I-signal and the Q-signal which are obtained by the quadrature demodulation processing, the I-signals and the Q-signals of respective subcarriers will not effectively undergo the amplitude error correction and the phase error correction, with the result that the characteristic of the received signal is not satisfactorily improved.
Accordingly, each of the device characteristics of the quadrature demodulation unit 104, the low-pass filters 105i and 105q, and the A/D converters 106i and 106q influences upon each subcarrier signal in a different manner. Therefore, when the amplitude error and the phase error between the I-signal and the Q-signal due to the device characteristics of these components are corrected, it is necessary to correct the amplitude error and the phase error between the I-signal and the Q-signal of each subcarrier for improving the received signal characteristic.